Solar generation and cogeneration systems can offer a logical alternative or addition to fossil fueled energy systems as fuel costs and environmental concerns increase. The solar heat that is collected in a collection system, with or without electricity (such as by way of photovoltaic cells), may provide a major boost to an energy system's value. Unfortunately, however, “solar cogeneration” systems need to be located at the site of use, which presents challenges to most existing or previous concentrator methods. Because the collected heat generally is at low temperature (e.g., typically 40-80 degrees C.), the heat energy cannot be transmitted far without substantial parasitic losses. Further, the capital cost of hot water and other heat transmission systems favors direct on-site use. And, such low temperature heat generally cannot be converted in a heat engine to mechanical or electrical power because of the small temperature differential versus ambient temperatures. Accordingly, systems are needed that harvest light energy and transfer the harvested energy easily to the heating requirements at the site of use, such that the immediate needs of the site are factored into how the system is controlled.
Solar cogeneration technologies are, in part, held back by challenges in creating optical systems that are both inexpensive and that can be mounted or integrated into a building. One problem is the practical limit for how tall a design can be to withstand forces from windy conditions on the device and building on which it may be mounted. Tying a cogeneration apparatus into the foundation or load bearing structure of a building creates expensive installations and/or mounting systems to accommodate system stresses, particularly on the roof. Many commercial sites lack sufficient ground space for a reasonably sized system, and roof-mounting is the only viable option to obtain sufficient collector area.
Efforts have been made to meet the foregoing challenges. For instance, MBC Ventures, Inc., the assignee of the instant application, has developed solar harvesting apparatus and methods and their incorporation into building structures, as described in co-owned U.S. Patent Publication No. US2009/0173375 titled “Solar Energy Conversion Devices and Systems” (U.S. application Ser. No. 12/349,728), and co-owned U.S. Patent Publication No. US2011/0214712 titled “Solar Energy Conversion” (U.S. application Ser. No. 13/056,487), both of which specifications are incorporated herein by reference in their entireties. While such systems provide significant improvement over prior solar harvesting systems, opportunities remain to enhance the reliability, reduce cost, and improve the performance of such systems.
Moreover, a skylight energy management system that improves upon such prior apparatus and methods are set forth in co-owned U.S. Patent Publication No. US2013/0199515 titled “Skylight Energy Management System” (U.S. application Ser. No. 13/749,053), the specification of which is incorporated herein by reference.
Nonetheless, a consideration with such prior apparatus, along with skylight fixtures previously known in the art, is the challenge they present in capturing both diffuse and ambient light, in addition to direct solar light. More particularly, skylights provide an opening in the roof of a building to allow natural light to enter a building, which can reduce the energy consumption required for artificial lighting as well as bring health and productivity benefits to the occupants. In order to maximize these benefits, it would be desirable for skylights to efficiently capture both direct solar and diffuse ambient light, and deliver light that is diffuse to the space below that provides even illumination and no unpleasant glare spots. Skylights mounted on flat-roofed commercial buildings are generally made of semi-transparent glass or plastic materials which in some cases have light attenuating and diffusing features added to them, such as coloration or prismatic lensing.
Skylights are generally mounted on a roof curb. The curb typically comprises a waterproofing and structural feature that fixes the skylight to the roof over the opening and maintains the waterproof seal from the perimeter of the skylight down to the roofing membrane. The curb must be structurally strong enough to withstand the wind- and snow-driven structural loads to which the skylight is subjected, and to transfer the loads to the roof structure. The curb must also be completely watertight so that any standing or flowing water or melting snow will not leak through the curb into the space below. Curbs are generally made of sheet metal, wood or other structural members and are generally flashed to the roofing material using similar materials as the roof membrane itself. Generally, all of these materials are opaque to the transmission light. The required height of the curb above the roof surface is a function of the local building codes, which are dependent on local climate and the type of roof. In climates with significant snow or high rainfall levels, the height of the curb above the roof surface can be as high as 12 to 14 inches. This implies that the light coming from the skylight must pass through a channel which impedes the flow of light from the skylight down into the space below. This space that exists from the bottom plane of the skylight down to the open area at the bottom of the curb is called the light well. Efficiency of transmission of light through the light well is a function of the geometry (the length and width of the walls) as well as the optical properties of the walls of the light well. Typical light well efficiencies range from 75% to close to 95% if highly reflective materials are used for the walls of the light well. These reflective materials add expense to the cost of the skylight without adding any additional light.
Furthermore, a significant fraction of the cooling load on a large flat roofed building is typically due to the solar heat gain on the roof surface, which is conducted through the structure into the space below. A common solution to reduce this heat load that has been introduced in recent years is to form the surface of the roof of a highly reflective material so that the top layer of the roof stays cooler and less heat is conducted into the building, thus significantly reducing the cooling load of the space below.
A current common commercial roofing practice employs highly reflective material made of thin rubber sheet that is coated on the top side with a white or silver highly reflective coating. The intended effect of these coatings is to simply reflect the sun's rays back to the sky and keep the heat out of the building. However, the bright diffuse light that is reflected from the surface of the building also represents an opportunity to provide additional light to a skylight that is mounted on such a roof. Many of these roof coatings have highly diffuse reflection, which means that the sun's rays that strike the roof are then scattered in all directions. A skylight that has a near-vertical surface area can thus receive a significant amount of diffuse light flux as a result of this reflected and scattered light. Over the course of a day, this flux can be 50 to 100% of the direct flux from the sky. In such a construction, this reflected light would strike the skylight from an angle below the horizon; however, existing skylights are not designed to capture light that strikes them from below.
Thus, there remains a need in the art for a simple, easy-to-install skylight construction that is capable of capturing and making use of direct solar along with both diffuse and ambient light, and moreover that can make use of light reflected off of the roof surface to add to the lighting function performed by such skylight.